1. Field of the Invention
The invention relates to amplifiers such as differential amplifiers. More particularly, the invention relates to amplifiers such as differential amplifiers having improved common mode voltage range (CMVR).
2. Description of the Related Art
Amplifiers such as differential amplifiers are useful for various circuit applications, including within signal receivers. Differential amplifiers typically are used to amplify both analog and digital signals, and to provide amplifier outputs in response to differential inputs. Furthermore, differential amplifiers are readily adaptable to function as an operational amplifier, a comparator, a sense amplifier and as a front-end buffer stage for another circuit.
However, conventional differential amplifiers will operate only over a relatively narrow range of common-mode input voltages. As the amplifier is forced to extend beyond this small range of common-mode voltages, the differential-mode gain drops off sharply and sometimes drops to zero. Another difficulty in conventional differential amplifiers is the possibility of a portion of the common mode voltage range (CMVR) disabling the functionality of the circuit. For example, if the input signals with a common mode voltage of 0 to V.sub.th are applied to N-channel transistors, the circuit can become disabled.
One method of extending the common mode range of differential amplifiers is disclosed in U.S. Pat. No. 4,958,133, issued Sep. 18, 1990 to Bazes and entitled "CMOS Complementary Self-Biased Differential Amplifier With Rail-To-Rail Common-Mode Input-Voltage Range". The Bazes differential amplifier combines two complementary folded-cascade differential amplifiers and connects all of the bias inputs to a single internal node to yield a differential amplifier that successfully extends the common voltage range without disabling the circuit. However, the Bazes' extension of the common-mode voltage range is at the expense of power, circuit design simplicity and performance limitation.
It is desirable to have a differential amplifier for amplifying differential input voltages in which the common-mode component can vary over an extremely wide range of voltages without otherwise causing amplifier operating difficulties.